1. Introduction to MATLAB
CS534 Fall 2015

2. What you'll be learning today
MATLAB basics (debugging, IDE)
Operators
Matrix indexing
Image I/O
Image display, plotting
A lot of demos
...

3. Who am I
Jia-Shen Boon
文嘉绅

4. Who am I

5. Contact
Office: 1302CS Office hours: Tuesdays and Thursdays 4:00 - 5:00 p.m., and by appointment

6. Accessing MATLAB Get a local copy from the Campus Software Library
Also available in the Linux and Windows labs
Also remotely accessible via ssh
On Linux, type matlab into the terminal

7. What's great about MATLAB
Matrices are treated as 1st class citizens
Effortless to inspect images and plots

8. What's not so great about MATLAB
Data structures besides matrices can feel like 2nd class citizens
Proprietary ($$)
Syntax sometimes not as elegant as other dynamically typed languages

9. Tip #1: Google is your friend.

10. MATLAB basics

11. Your first MATLAB command
Arithmetic operators + - * /
Assignment operator =
>> total = 1 + 1;
3. Semicolon suppresses output
2. Value from (1) is assigned to this variable
1. MATLAB computes what's 1 + 1

12. MATLAB IDE Current Path COMMAND WINDOW Where you type commands
Workspace
List of your current variables
Filespace
Command History
List of previous commands

13. Demo

14. What is a matrix? 5 3 4
3 6 8
1x3 vector
3x1 vector
1 2 3
4 5 6
MxNxP matrix
2x3 matrix
Terms: row, column, element, dimension

15. What is a matrix?
Second dimension
1 2 3
4 5 6
First dimension

16. What is a matrix? An RGB image is a 3D MxNx3 matrix 236 252 255 24 7278
An RGB image is a 3D MxNx3 matrix

17. The "layers" in a color image are often called channels
red channel
green channel
blue channel

18. Expliciting defining a matrix
semicolon separates rows
Bonus: you can define a matrix using other matrices too! What's the output of the following?
a = [1 2]; b = [a 3 4]; c = [5 6 a; b]; disp(c);

19. Expliciting defining a matrix (cont'd)
>> A = 1 : 2 : 10
Colon creates regularly spaced vectors
start value
end value
increment

20. Demo

21. More arithmetic operators
+ Addition
- Subtraction
* Matrix Multiplication
^ Matrix Power
' Transpose
\ Left Matrix Division (Solves A*x=B)
/ Right Matrix Division (Solves x*A=B)
.* Element by Element Multiplication
./ Element by Element Division
.^ Element by Element Power

22. How Operations Work 1 2 3 4 A = 3 1 5 6 B = 4 3 8 10 A+B = -2 1 -2 -2
1 2
3 4
A =
3 1
5 6
B =
4 3
8 10
A+B =
-2 1
-2 -2
A-B =
13 13
29 27
A*B =
A^2 =
A\B =
A/B =
solves A*x = B
solves x*A = B

23. Element-wise operations
1 2
3 4
A =
3 1
5 6
B =
A ./ B =
15 24
A .* B =
A .^ B =

24. Transpose
9 11
13 15
C =
C’ =

25. Demo

26. size() 1 2 3 4 5 6 >> A = [1 2 3; 4 5 6]; >> size(A, 1)
ans = 2
>> size(A, 2) 3 A
asks for first dimension
asks for second dimension

27. size() cont'd 1 2 3 4 5 6 >> A = [1 2 3; 4 5 6];
>> [height, width] = size(A)
height = 2
width = 3 A

28. Matrix indexing 1 2 3 4 5 6 A(2, 3) Element on 2nd row, 3rd column
Note: indexing starts from 1, not zero!

29. Matrix indexing (cont'd)
1 2 3
4 5 6
A(2, :)
Returns 2nd row
A(:, 3)
Returns 3rd column

30. Matrix indexing (cont'd)
1 2 3
4 5 6
>> A = [1 2 3; 4 5 6];
>> A(:, 2:end)
ans =
Returns concatenation of 2nd, 3rd, …, last column

31. Matrix indexing (cont'd)
1 2 3
4 5 6
>> A = [1 2 3; 4 5 6];
>> A(:, [1 end 1])
ans =
Returns concatenation of 1st, last and 1st column

32. Indexing rules apply to 3D matrices too
Element on 2nd row, 4th column of the 3rd channel

33. Logical operators == is equal to
< > <= >= less/greater than
~ not
~= not equal to
& logical AND
| logical OR
&& short-circuit AND
|| short-circuit OR
Cover logical indexing?

34. Demo

35. Flow control

36. end keyword Instead of using brackets, MATLAB uses “end” C MATLAB
for(int a=0;a<=10;a++){
if( a>3){
... … } C
for (a=0:10)
if (a>3)
... …
end
MATLAB

37. if/elseif/else if (boolean) … elseif (boolean) else end
Notice elseif is one word

38. while-loop while expression statement end A = 0; while A < 5
disp(A);
A = A + 1;

39. for-loop for index = values statements end
Note: for-"condition" overwrites changes to index within the loop!

40. for-loop (cont'd) for A = 1 : 2 : 8 disp(A); end
Bonus: values can be a 2D matrix too! What is the output of the following?
for A = [1 2 3; 4 5 6]; disp(A); end;

41. Tip #2: Think in terms of matrices.

42. Logical and linear indexing
Fancier ways to index a matrix.

43. Question: What's the sum of all elements greater than 2 in A?
1 2 3
4 5 6
summation = 0;
[height, width] = size(A);
for j = 1 : height
for i = 1 : width
if A(j, i) > 2
summation=summation+A(j, i);
end
Motivating example for logical indexing

44. Question: What's the sum of all elements greater than 2 in A?
1 2 3
4 5 6
summation = sum( A (A > 2) );
0 0 1
1 1 1 T
Motivating example for logical indexing 22

45. A closer look at sum(A(A>2))
1 2 3
4 5 6
A > 2 returns logical matrix[0 0 1; 1 1 1]
A([0 0 1; 1 1 1])returns column vector [ ]'
This syntax is known as logical indexing.
A, a MxN matrix is indexed by a MxN logical matrix.

46. More examples of logical indexing
Logical indexing on the right-hand side
count = sum(2 < A | A == 5);
ave = mean(A(mod(A, 2) == 0));
Logical indexing on the left-hand side
A(isnan(A)) = 0;
count is the sum of elements in A that are less than 2 or are equal to 5.
ave is the average of all elements in A that are a multiple of 2.
Final example assigns all nan in A to 0.

47. Linear indexing
Indexes an element in a matrix with a single number, in column-major order.
A(5)
A(1)
A(3)
A =
A(6)
A(2)
A(4)

48. Statistical functions usually return one statistic per column
>> sum(A)
ans =
>> sum(A(:)) 21

49. List of column-wise statistical functions
sum mean max min median std var ...
sum(A, 2) returns row-wise statistics instead

50. Vectorized code tends to be faster than non-vectorized code
A = rand(1000,1000);
B = rand(1000,1000);
for i = 1:size(A,1),
for j = 1:size(A,2),
C(i,j) = A(i,j) + B(i,j);
end
Using loop: Elapsed time is seconds. 50

51. Vectorized code tends to be faster than non-vectorized code
C = A + B;
Elapsed time is seconds. 51

52. MATLAB file types

53. There's two types of .m files
Matlab code is saved in .m files
Function .m files
Contain a function definition
One function per file
FILE NAME MUST MATCH FUNCTION NAME
Script .m files
Contain a list of commands
Can be named anything
Often used as drivers for functions you have implemented
Kind of like main in other languages

54. Writing MATLAB functions
Structure of a MATLAB function
Functions Can Return Multiple values
Make sure you initialize your return variables
function returnVal = FunctionName (input1,input2)
%Adds two numbers
returnVal = input1+input2;
end
function [return1, return2] = FunctionName (input1,input2)
return1 = input1+input2;
return2= 0;
end

55. How do these two files behave differently?
%average1.m
total = x + y;
average = total / 2;
%average2.m
function average = average2(x, y)
total = x + y;
average = total / 2;

56. Matrices are effectively passed into functions "by value"
%my_script.m
vector = [ ];
disp(vector) % (1)
sort(vector);
disp(vector) % same output as (1)

57. Matrices are effectively passed into functions "by value"
%my_corrected_script.m
vector = [ ];
sorted_vector = sort(vector);

58. Debugging

59. Before you enter debugging mode
Click this to run program. Program pauses at checkpoints, if there's any.
Click along this column to set/remove breakpoints
Check this option for the program to pause once an error occurs

60. Debugging mode works like any other IDE

61. Data types

62. Important data types logical true/false uint8 8-bit unsigned integer
double 64-bit double-precision floating point
single 32-bit single-precision floating point
There's also uint16, uint32, int8, int16...

63. Be aware of your return types!
imfilter()
imread()

64. Be aware of your argument types!
imhist()
interp2()

65. Is there anything wrong with this script?
im_original = imread('lena_gray.jpg');
[Xq, Yq] = meshgrid(0.5:99.5, 0.5:99.5);
im_interp = interp2(im_original, Xq, Yq);
Answer: interp2 expects im_original to be a single or double but imread returns a logical, uint8 or uint16. Therefore this script will always throw an error.

66. Demo

67. Images

68. Generic image processing script
filename = 'badgers.jpg';
im_orig = imread(filename);
im_processed = my_func(im_orig);
figure; imshow(im_orig);
figure; imshow(im_processed);
imwrite(im_processed, 'hw.jpg');

69. Important image related functions
imread Read image from disk
imwrite Write image to disk
figure Create new figure
imshow Display image
im2double Convert image datatype to double
im2uint8 Convert image datatype to uint8

70. Histogram of each channel

71. Histogram of each channel
im = imread('badgers.jpg');
for channel = 1:3
subplot(2, 3, channel);
imshow(im(:, :, channel));
subplot(2, 3, channel + 3);
imhist(im(:, :, channel));
end
imhist expects a 2D matrix!

72. subplot() squeezes more into a figure
subplot(m, n, p);
p determines location of this subplot; it's an index in row-major order
p is based on an m x n grid 1 2 3 4 5 6

73. Useful MATLAB "hacks"

74. Problem - too many windows

75. Solution - dock figures
set(0, 'DefaultFigureWindowStyle', 'docked')

76. How to time your code tic; % stopwatch starts here
%-- Do some stuff here --%
toc; % print time elapsed since tic

77. disp() without newline
%Script
disp('newline always included.');
fprintf('No newline here.');
fprintf(' Here is a newline.\n');

78. Reference https://www.google.com/
MATLAB cheat sheet
This presentation

79. fin.